This proposal is for continuation of funding of the National Biomedical Electron Paramagnetic Resonance (EPR) Research Resource for years 27 through 31. This Research Resource is broadly based, with unique instrumentation in many branches of EPR spectroscopy. Eight faculty members with EPR training enhance collaborations. Technological Research and Development (TR & D) contains two Sections: Device Design Driven and Methodological Development. In the first, it is proposed to purchase a Bruker W-Band accessory to an ELEXSYS EPR spectrometer and to enhance the high frequency capability by improved frequency translation technology, which will permit use of advanced EPR methods, including time-locked sub-sampling (TLSS) detection and multiquantum EPR at high field strength. Enhancement of W-band capabilities in the liquid phase is also proposed. In addition, three new categories of resonators were discovered in the previous funding period, and it is proposed to develop practical EPR structures based on these discoveries. The second Section of TR&D contains four sub-sections: Methodology Development at W-band, Site- Directed Spin Labeling using Saturation Recovery as well as W-band, Spin Trapping Using Loop Gap Resonators at X- and Q-band, and Measurement of Rapid Reactions Using Loop Gap Resonators with the ELEXSYS X-band Spectrometer. Seven specific Collaborations are described, each written by a different EPR Center faculty member, together with one of their colleagues from another institution or department. The existing young investigator Training program will be continued. Postdoctoral fellows will be appointed for enhanced Training at the EPR Center. A workshop on advanced EPR instrumentation will be held as part of the Dissemination activities of the Center. A special Service project is proposed: development of a YIG oscillator to replace klystrons. EPR spectroscopy is used to study biomolecular structure and function, including spin-labeled macromolecules, metalloproteins, and free radicals. The mission of the Resource is to serve the community of EPR spectroscopists with emphasis on development of advanced EPR instrumentation and new EPR methodology.